AURORA BOREALIS AND AURORA AUSTRALIS.
These splendid meteors are generally considered as the result of a combination of the two powers of magnetism and electricity. When the light, or aurora, appears chiefly in the north part of the heavens, it is called the aurora borealis, or northern lights; and when chiefly in the south part, the aurora australis, or southern lights. Where the coruscation is more than ordinarily bright and streaming, which, however, seldom occurs in the north, it is denominated lumen boreale; and where these streams have assumed a decided curvature, like that of the rainbow, they are distinguished by the name of luminous arches.
The aurora is chiefly visible in the winter season, and in cold weather. It is usually of a reddish color, inclining to yellow, and sends out frequent coruscations of pale light, which seem to rise from the horizon in a pyramidal, undulating form, shooting with great velocity up to the zenith. It never appears near the equator; but of late years has frequently been seen toward the south pole. The aurora borealis has appeared at some periods more frequently than at others. This phenomenon was so rare in England, or so little regarded, that its appearance was not recorded in the English annals between a remarkable one observed on the fourteenth of November, 1554, and a very brilliant one on the sixth of March, 1716, and the two succeeding nights, but which was much strongest on the first night. Hence it may be inferred, that the state of either the air or earth, or perhaps of both, is not at all times fitted for its production.
The extent of these appearances is surprisingly great. The very brilliant one referred to above was visible from the west of Ireland to the confines of Russia, and the east of Poland, extending over, at the least, thirty degrees of longitude, and, from about the fiftieth degree of latitude, over almost all the northern part of Europe. In every place, it exhibited, at the same time, the same wonderful features. The elevation of these lights is equally surprising: an aurora borealis which appeared on the sixteenth of December, 1737, was ascertained, by means of thirty computations, to have an average hight from the earth of one hundred and seventy-five leagues, equal to four hundred and sixty-four English miles.
Captain Cook, in his first voyage round the world, observed that these coruscations are frequently visible in southern latitudes. On the sixteenth of September, 1770, he witnessed an appearance of this kind about ten o’clock at night, consisting of a dull, reddish light, and extending about twenty degrees above the horizon. Its extent was very different at different times, but it was never less than eight or ten points of the compass. Rays of light, of a brighter color, passed through and without it; and these rays vanished and were renewed nearly in the same time as those in the aurora borealis, but had little or no vibration. Its body bore south-south-east from the ship, and continued, without any diminution of its brightness, till twelve o’clock, when the observers retired. The ship was at this time within the tropic of Capricorn.
On the seventeenth of February, 1773, during his second voyage, Captain Cook speaks of a beautiful phenomenon that was observed in the heavens. “It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and spreading gradually over the whole southern parts of the sky. These columns even sometimes bent sideways at their upper extremity; and, although in most respects similar to the northern lights, (the aurora borealis of our hemisphere,) yet differed from them in being always of a whitish color; whereas ours assume various tints, especially those of a fiery and purple hue. The stars were sometimes hidden by, and sometimes faintly to be seen through the substance of these southern lights, aurora australis. The sky was generally clear when they appeared, and the air sharp and cold, the mercury in the thermometer standing at the freezing-point; the ship being then in fifty-eight degrees south.” On six different nights of the following month (March) the same phenomenon was observed.